The invention relates to a method of manufacturing an electron gun for use in a colour display tube, wherein a number of electrodes are beaded by securing them to beading rods to form a beaded unit which sequentially comprises, viewed in the propagation direction of the electrons, a beam forming section and a main focusing section, the main focusing section having a focus electrode and an anode electrode, in a further step a centring cup being coupled to the anode electrode.
The invention further relates to an electron gun for use in a colour display tube, having at least one cathode, a beaded unit comprising, sequentially, viewed in the propagation direction of the electrons, a number of electrodes, having a beam forming section and a main focusing section, the main focusing section having a focus electrode and an anode electrode, the electron gun further comprising a centring cup which is coupled to the anode electrode.
The invention also relates to a colour display tube provided with such an electron gun.
The electron gun as described in the opening paragraph is disclosed in U.S. Pat. No. 4,678,964. In this patent specification an electron gun is described that is provided with a field correction element. This element is connected to the centring cup and this sub-assembly is connectedxe2x80x94by weldingxe2x80x94to the beaded unit which contains the electrodes of the beam forming section and the main focusing section. The main function of this field correction element is to tune the astigmatism of centre and side beams of the electron gun. Additionally, this field correction element may be used for carrying out convergence corrections.
The electron gun as disclosed in U.S. Pat. No. 4,678,964 has the disadvantage that the manufacturing method for assembling the field correction element lacks the accuracy that is needed for fully utilizing the possibilities of the field correction element. In customary electron guns, the field correction element is a deep drawn part that is welded to thexe2x80x94also deepdrawnxe2x80x94centring cup. Subsequently, this sub-assembly is connected to the anode electrode of the main focusing section by welding them together. This method leads to relatively large positional inaccuracies of the field correction electrode with respect to its distance to the main focusing section and its rotation around the longitudinal axis of the electron gun.
It is an object of the present invention to provide a method of manufacturing an electron gun in which the positional accuracy of the field correction element is significantly improved compared to the electron gun from the prior art.
According to the invention this object is achieved by a method of manufacturing which is characterized in that the main focusing section further comprises an additional electrode positioned in between the anode electrode and the centring cup, which electrode is mounted in the beaded unit.
The invention is based on the recognition that the positional accuracy of the field correction element is greatly improved if, during the manufacturing process, this additional electrode is treated in a similar way as all the other electrodes making up the beam forming section and the main focusing section of the electron gun. This implies that the additional electrode will be part of the beaded unit. During the beading process the electrodes are positioned by stringing them together on centring pins which penetrate the apertures or by providing the electrodes with an outside reference that enables a fixation of the electrodes along their circumference. The spacing between the electrodes is secured by placing spacers between the different electrodes during the beading process, which spacers are removed after the electrodes are secured to each other by the beading rods. This process yields beaded units for electron guns with a very high degree of accuracy regarding the spacing between the different electrodes and the rotation of the individual electrodes.
In a preferred embodiment the additional electrode is electrically connected to the anode electrode.
By connecting the additional electrode to the anode electrode, for instance by interconnecting these two electrodes by means of a small wire or tape, the electron gun can be applied in a similar way as the prior art electron guns, and it does not require an additional voltage for driving the additional electrode.
A further embodiment is characterized in that the additional electrode comprises rectangular apertures.
In practice it has been found that, if the combination of main focusing section and additional electrode is optimized, a simple structure of rectangular apertures for the additional electrode will generally be able to fulfil the requirements.
In a further embodiment, the anode electrode has an apertured plane, and the distance between the additional electrode and the apertured plane of the anode electrode is less than 1.5 mm.
In order to be effective, the additional electrode has to be positioned in the vicinity of the anode electrode, because it has to influence the electric field penetrating the apertures of the anode electrode. In practice, it has been found that the additional electrode loses its effectiveness when the distance to the apertured plane of the anode electrode becomes larger than 1.5 mm.
In a still further embodiment the distance between the additional electrode and the apertured plane of the anode electrode is 0.7 mm.
This distance enables the electric field behind the main focusing section to be influenced in a proper and effective way, while a still smaller distance leads to changes in the electric field of the main focusing section itself. This will deteriorate the characteristics of the main focusing section.
The invention further relates to an electron gun whose properties are obtained by employing the above-described method of manufacturing an electron gun, and to a colour display tube provided with such an electron gun.
These and other aspects of the invention will be apparent from and elucidated by way of non-limitative examples with reference to the drawings and the embodiment(s) described hereinafter.